This invention relates to self-hardening mold sand with a comparatively small moisture content, yet having high surface stability and compressive strength.
Prior artisans have provided self-hardening binders for mold sand which harden fast even at low temperature. These binders are made by adding insoluble anhydrite to clinker powder containing calcium haloaluminate of 11CaO.7Al.sub.2 O.sub.3.CaX.sub.2 (X refers to halogen atom).
Now mold sands for casting are being manufactured by mixing binder, or binder with additives such as various kinds of sulphate (for ex. sodium sulphate, calcium sulphate, hemihydrate) and/or with carboxylic acid or boric acid, with fire-resistant granules such as silica sand or olivine sand to develop strength of the sand in a short time.
To facilitate pounding of such mold sands thoroughly, a proper volume of water should be added to achieve proper compressive strength. If too much water remains, casting will not be effected well due to occurrence of "blow" at the time of casting material. Also the stability of mold surface will become low after cured due to "brittle binding" of sand. This causes "scab" and "sand burning."
Water also enhances surface stability. The improvement of surface stability of mold has been attempted for example by increasing binder to enhance the strength of sand or by adding starch to the sand. Yet such was not satisfactory with respect to permeability of the mold, mositure content and fire-resistance of the mold sand.